The 8.1 update to Windows RT will be delivered via the Windows Store, however it is likely that a OS patch will first be required in order to allow the OS to be updated in this way - so, you'd first get an OS update delivered via Windows Update in the usual manner, this would then 'unlock' the 8.1 RT update in the Windows Store for installation. These updates are not year released, but are available only in preview form. You can get the preview for your Surface RT device via the Microsoft website, however please note it is a preview at the moment, so only install it if you are happy running a preview release Operating System.
You can probably hack your Surface RT to run a different OS yes (you can already jailbreak Windows RT on the surface), but you probably want to make sure you have a suitable 'touch enabled' OS so you can work with it appropriately, and make sure you have a 'way back'.
My problems with Windows 8.1 RT Preview - Please, if you have any information to help on these issues, reply:
1) Error on IE11 (Error: possible error with .gwt.xml module file...) - many sites return this error, but I can keep using after clicking OK. Sites like BlogSpot.com also mentions features that could not work with the browser (but couldn't find any in fact)
2) Windows + S screen clipping (feature from OneNote) now opens the Search Charm. Don't know not which key is used for OneNote Clipping
3) Couldn't find how to synchronize with Outlook RT with Live.com/Outlook.com/Exchange so I can have all appointments, contacts and tasks sync with other devices/computers
4) Some apps are not behaving like before. Some minor screen weird behavior, some apps are slower now, OneNote copy and paste from one note to another sometimes is very slow
5) When I check manually Apps Update, it always bring the Windows 8.1 new apps to be installed as it was not installed before...it happens also with some other 3rd party apps